The present invention relates to communications networks. More particularly, and not by way of limitation, the present invention is directed to a system and method of modulation and coding scheme adjustment for a Long Term Evolution (LTE) shared Data Channel. Fast link adaptation to the fading channel conditions is adopted in modern wireless communications (e.g., Universal Mobile Telecommunications Systems (UMTS), LTE, and future evolutions) to enhance system throughput capacity as well as user experience and quality of services. Crucial to the working of fast link adaptation is the timely update of channel conditions that is fed back from the receiver to the transmitter. The feedback can take on several related forms, such as signal to noise ratio (SNR), signal to interference and noise ratio (SINR), received signal level (power or strength), supportable data rates, supportable combination of modulation and coding rates, to supportable throughputs. The information may also pertain to an entire frequency band, such as in Wideband Code Division Multiple Access (W-CDMA) or in a specific portion of the frequency band as made possible by systems based upon orthogonal frequency division multiplexing (OFDM), such as the LTE system. The generic term “channel quality indicator” (CQI) is used to refer to any such feedback messages.
FIG. 1 illustrates a simplified block diagram of a UMTS network 100 that comprises a 3rd Generation (3 G) network referred to as a core network 102 and a UMTS Terrestrial Radio Access Network (UTRAN) 104. The UTRAN comprises a plurality of Radio Networks Controllers (RNCs) 106. In addition, there is a plurality of RNCs performing various roles. Each RNC is connected to a set of base stations. A base station is often called a Node-B. Each Node-B 108 is responsible for communication with one or more User Equipments (UEs) or mobile station 110 within a given geographical cell. The serving RNC is responsible for routing user and signaling data between a Node-B and the core network.
In the downlink data operations of the LTE system, the CQI messages are fed back from the mobile station 110 to the base station (e.g., Node-B 108) to assist the transmitter on the decision of radio resource allocation. The feedback information may be used to determine transmission scheduling among multiple receivers, to select suitable transmission schemes (such as the number of transmit antennas to activate), to allocate the appropriate amount of bandwidth, and to form supportable modulation and coding rates for the intended receiver. In the uplink data operations of the LTE system, the base station (e.g., Node-B 108) can estimate the channel quality from the demodulation reference symbols or the sounding reference symbols transmitted by the mobile stations.
The range of CQI report message for LTE systems is shown in Table 1. The CQI table has been specifically designed to support modulation and coding scheme (MCS) adaptation over wide-band wireless communication channels. The transition points from a lower-order modulation to a higher-order modulation have been verified with extensive link performance evaluation. These specific transition points between different modulations thus provide guideline for optimal system operation.
TABLE 14-bit CQI Table for LTECQICodingSpectral efficiencyindexModulationrate × 1024(bits per symbol)0out of range1QPSK780.152QPSK1200.233QPSK1930.384QPSK3080.605QPSK4490.886QPSK6021.18716QAM3781.48816QAM4901.91916QAM6162.471064QAM4662.731164QAM5673.321264QAM6663.901364QAM7724.521464QAM8735.121564QAM9485.55
Based on the CQI reports from a mobile station, a base station may choose the best MCS to transmit data on the physical downlink shared channel (PDSCH). The MCS information is conveyed to the selected mobile station in the 5-bit “modulation and coding scheme” field (IMCS) of the downlink control information. As shown in Table 2 below, the MCS field signals to the mobile station both the modulation and the transport block size (TBS) index. In conjunction with the total number of allocated resource blocks, the TBS index further determines the exact transport block size used in the PDSCH transmission. The last three MCS entries are for hybrid automatic request (HARQ) retransmissions and, thus, the TBS remains the same as the original transmission.
TABLE 2Modulation and transport block size index table for LTE PDSCHMCS IndexModulationTransport block(IMCS)(Qm)size index (ITBS)0QPSK01QPSK12QPSK23QPSK34QPSK45QPSK56QPSK67QPSK78QPSK89QPSK91016QAM91116QAM101216QAM111316QAM121416QAM131516QAM141616QAM151764QAM151864QAM161964QAM172064QAM182164QAM192264QAM202364QAM212464QAM222564QAM232664QAM242764QAM252864QAM2629QPSKreserved3016QAM3164QAM
The specific TBSs for different number of allocated radio blocks are listed in a large 27×110 table in 3GPP Technical Specification 36.213. However, these TBSs are designed to achieve spectral efficiencies matching the CQI reports. More specifically, the TBSs are selected to achieve the spectral efficiencies shown in Table 3. Note that the CQI report table and, consequently, the MCS and TBS tables are designed based on the assumption that 11 OFDM symbols are available for PDSCH transmission. Thus, when the actual number of available OFDM symbols for PDSCH is different than 11, the spectral efficiency of the transmission will deviate from those shown in Table 3.
TABLE 3Spectral efficiency target for LTEwith 11 OFDM symbols for PDSCHMCS IndexModulationSpectral efficiency(IMCS)(Qm)(bits per symbol)0QPSK0.231QPSK0.312QPSK0.383QPSK0.494QPSK0.605QPSK0.746QPSK0.887QPSK1.038QPSK1.189QPSK1.331016QAM1.331116QAM1.481216QAM1.701316QAM1.911416QAM2.161516QAM2.411616QAM2.571764QAM2.571864QAM2.731964QAM3.032064QAM3.322164QAM3.612264QAM3.902364QAM4.212464QAM4.522564QAM4.822664QAM5.122764QAM5.332864QAM6.25
The LTE system has been designed to support a wide range of operation modes including the frequency division duplex (TDD) and time division duplex (TDD) modes. Each of these modes can also operate with normal cyclic prefix (CP) lengths for typical cell sizes or with extended CP lengths for large cell sizes. To facilitate downlink to uplink switching, some special TDD subframes are configured to transmit user data in the Downlink Pilot Time Slot (DwPTS) with shortened duration. Furthermore, the system can dynamically appropriate available resources between control information and user data information. For instance, the radio resource in a normal subframe is organized into 14 OFDM symbols. The system can dynamically use 1-3 OFDM symbols or 2-4 OFDM symbols in case of very small system bandwidths to transmit control information. As a result, the actual number of OFDM symbols available for data transmission is 13, 12, 11 or 10. A complete summary of the number of available OFDM symbols for PDSCH transmission in different operation modes is given in Table 4 below.
TABLE 4Available number of OFDM symbols for PDSCH (NOS) in LTENumber of OFDM symbolsfor control informationOperation mode1234FDD, TDDNormal CP13121110Extended CP111098TDD DwPTSconfigurations8765normal CP1, 6configurations98762, 7configurations109873, 8configuration1110984TDD DwPTSconfigurations7654extended CP1, 5configurations87652, 6configuration98763
The CQI report table and, consequently, the MCS tables discussed above are designed based on the assumption that 11 OFDM symbols are available for PDSCH transmission. As can be observed in Table 4, there are many cases where the actual resource available for transmission does not matched this assumption. The impact of this mismatch is illustrated in Table 5 below.
TABLE 5Code rate with different number of OFDM symbols for PDSCH in LTE  
The code rate becomes excessively high when the actual number of OFDM symbols for PDSCH is substantially less than the assumed 11 symbols. These cases are highlighted with the dark shading in Table 5. Since the mobile station will not be able to decode such high code rates, transmissions based on these dark shaded MCSs will fail and retransmissions will be needed.
In addition, with the mismatch of radio resource assumption, code rates for some of the MCSs deviate out of the optimal range for the wideband wireless system. Based on extensive link performance evaluation, the CQI reports in Table 1 have been designed based on the following principles. The code rates for Quadrature Phase Shift Keying (QPSK) and 16 Quadtrature Amplitude Modulation (16QAM) should not be higher than 0.70. Furthermore, the code rates for 16QAM and 64 Quadtrature Amplitude Modulation (64QAM) should not be lower than 0.32 and 0.40, respectively. As illustrated with the light shading in Table 5, some of the MCSs result in sub-optimal code rate.
Since data throughput is reduced when transmissions are based on unsuitable sub-optimal code rates, a good scheduling implementation in the base station should avoid using any shaded MCSs shown in Table 5. It can be concluded that the number of usable MCSs shrink significantly when the actual number of OFDM symbols for PDSCH deviates from the assumed 11 symbols. It should also be noted that some of the unusable MCSs are locate in the middle of the MCS index range. This can complicate the operations of the scheduling algorithms in the base station.
A proposal to remedy these problems has been suggested. It was proposed to modify the TBS when the actual number of OFDM symbols for PDSCH having less than 11 symbols to bring the code rate into the suitable range. This proposal is undesirable because it introduces additional complications to the operation of the system. Most importantly, data packets can be retransmitted in subframes with different number of available OFDM symbols than those in the initial transmissions. The proposed TBS modification thus reduces control information reliability and restricts scheduling flexibility of retransmissions. Furthermore, for allocations with small number of radio blocks, the proposed modification algorithm is ineffective in the code rate adjustment.